Two cases of Philadelphia chromosome positive chronic myelogenous leukemia (CML) demonstrated ring chromosomes. The appearance of the ring coincided with evolution from the stable to the aggressive phase. A literature search yielded six other cases of ring chromosomes in CML; all were in or were entering the aggressive phase of the disease. Thus, as is the case with acute nonlymphocytic leukemia, in CML the finding of an acquired ring chromosome is associated with a poor prognosis.